1. Field of the Invention
This invention relates to an optical high density recording device capable of recording on a circular disc an image signal in physical shape at high density. More particularly, it is concerned with an optical high density recording device which carries out recording of a beam modulated by an image signal onto a photosensitive disc.
2. Description of Prior Arts
There has already been proposed means for reproducing video-signals, wherein irregular waveforms which have been obtained by frequency-modulation or pulse-frequency-modulation are first recorded on a plastic disc with a spiral locus or path, then a needle or stylus is contacted under appropriate pressure along the recorded groove in the spiral locus having therein irregular surfaces corresponding to the irregular waveforms, while rotating the circular plastic disc, and vibrations which the stylus receives during its running along the irregular surfaces in the spiral groove are converted into an electrical signal by means of, for example, a ceramic piezo-electric element, and finally the electrical signal is demodulated to obtain reproduced video signal.
In general, a band of the video signal for use in the television technique should at least be 3 MHz, and, when such video signal is to be frequency-modulated, approximately 4 to 6 MHz is said to be required as the frequency range of the carrier wave. This frequency is higher than the upper limit of the general voice frequency, i.e., 20 KHz, by 200 to 300 times.
In the general audio-recording, when a master record disc, which is usually called "lacquer disc," is to be cut, the cutting is done at the same speed as the reproduction speed of the record, i.e., in the actual reproducing time. The reason for this is that, at the present level of the technique, a disc cutter can be driven relatively easily, even when the frequency is approximately equal to 20 KHz or twice as high as such voice frequency. However, in the case of the video signal, when the cutting is to be carried out in the actual reproducing time, the disc cutter must be driven in such high frequency as 4 to 6 MHz, in consequence of which, even if the depth of cutting is as shallow as 1 micron or less, it is hardly possible to carry out the cutting operation with the present-day technique.
At present, the only effective means for cutting the disc in the actual reproducing time in conformity to the irregularities on the groove surface of such high frequency as mentioned above is the recording of such high frequency signal onto a thin photoresist layer by means of a laser beam.
However, when the recording of the video signal is to be conducted on a disc coated with a photosensitive material with a light beam such as a laser beam which has been modulated by the video signal to be recorded, there inevitably takes place undesirable results for the reasons to be mentioned later. That is, when the wavelength of the video signal is long, the topmost and bottommost parts of the wavy surfaces in the groove formed by the beam irradiation become high, and, when the wavelength is short, the top and bottom parts of the wavy surfaces in the groove formed by the beam irradiation on the disc becomes low.
On the other hand, as a pickup to detect the undulating irregular surfaces formed in the groove on the disc and to reproduce the detected signal is for detecting intervals between the adjacent peak portions of the undulating irregular surfaces in the groove formed on the disc and reproducing the thus detected video signals, if the height of the peak and valley of the undulating surfaces are varied by frequency of the recorded video signal, the pickup slides over the disc at the portion where the variation in height exists, or the drop-out portion will inevitably be included in the reproduced video signal due to the pickup being depressed with a very large pressure, or, at the worst of the case, the disc is destroyed locally.